The Barometer of Sustainability is the only performance scale that measures human and ecosystem wellbeing together without submerging one in the other. The Barometer’s key features are:
• Two axes, one for human wellbeing, the other for ecosystem wellbeing. This enables each set of indicators to be combined independently, keeping them separate to allow analysis of people-ecosystem interactions.
• The axis with the lower score overrides the other axis in the analysis. This prevents a high score for human wellbeing from offsetting a low score for ecosystem wellbeing, or vice versa. This approach reflects the view that people and the ecosystem are equally important and that sustainable development must improve and maintain the wellbeing of both.

100 percent sustainability is a perfect state that is practically unattainable by anybody or any system. No matter how good a person or system is, there is always a sustainability deficit that cannot be overcome, as entropy affects living systems and their physical habitats without exceptions. This means that there is always room for improvement. Different persons or systems are located at different levels on the curve, with larger or smaller sustainability deficits, but with deficits all the same.

Imagine all of Earth’s chemistry as a mail sorter’s wall of letter slots in a post office, with the network of compartments extending toward infinity (see the bottom figure, next page). Each compartment represents a separate chemistry so that, for example, thousands of compartments are associated with stratospheric chemistry or with a human cell. An environmentally mobile persistent pollutant can move from compartment to compartment, sampling a large number and finding those compartments that it can perturb. Many perturbations may be inconsequential, but others can cause unforeseen catastrophes, such as the ozone hole or some of the manifestations of endocrine disruption. Most compartments remain unidentified and even for known compartments, the interactions of the pollutant with the compartment’s contents can usually not be foreseen, giving ample reason for scientific humility when considering the safety of persistent mobile compounds.